This invention relates generally to a process for making and depositing thin films of predominantly carbon with a gas phase plasma and ion beam conducted in a vacuum chamber and the resulting thin carbon films. The deposited carbon can possess many of the properties of diamond, of graphite, of hydrocarbon polymers or can have combined properties of, or intermediate properties, between diamond, graphite and polymers.
Procedures which are known for depositing thin carbon films on substrates include ion beam sputter deposition, magnetron sputtering, electron beam evaporation, plasma dissociation and others. Moreover, U.S. Pat. Nos. 4,541,890 (Cuomo et al) and 4,862,032 (Kaufman et al) each disclose end-Hall ion source systems for generating plasmas and extracting ions, the subject matter of which is incorporated herein by reference.
Amorphous carbon-based materials (a-C), the most prominent member of which is diamondlike carbon (DLC), have been described by M. Tamor in "Diamond and Diamondlike Carbon", Encycl. Appl. Phys. 5, pp. 1-24 (1993). According to Tamor, DLC is not a single material but is a broad class of materials which has properties suggestive of diamond, such as, high mechanical hardness, low coefficient of friction in sliding couple with steel, high dielectric constant, high electrical resistivity, chemical inertness and high transmittance at visible wavelengths. However, the measured value of each of these properties has been found to be continuously variable over quite a large range in DLC specimens that have been prepared in slightly different fashions. The variability in properties has been attributed to differences in the type of chemical bonding adopted by carbon in the materials. The range of property variations while retaining the same elemental composition varies greatly from fully electrically insulating to near-graphitic conductivity, such that, the name DLC does not adequately describe the class of materials. The term DLC is merely descriptive of a property of the harder members of the class whereas a-C is more representative of carbon-based materials that are deposited under conditions of kinetically energetic atomic or molecular bombardment during the film growth, particularly electrically accelerated ion bombardment. Thus, a-C would cover both amorphous and/or polyphase carbon with phase domain sizes on the order of nanometers wherein the main distinction between phases is the carbon chemical bond type. Although fully accepted nomenclature for the entire class has not been adequately developed, Tamor describes the plethora of nomenclature that does exist.
As used herein, the designations a-C and a-C:H, refer to amorphous carbon and amorphous carbon/hydrogen, both of which are believed to be deposited in the presence of energetic bombardment.
Categorization of a-C and a-C:H materials has been attempted by its properties, by the deposition method used, by hydrogen content, and by chemical bonding type. Thus, Aisenberg et al in "Ion Beam and Ion-Assisted Deposition of Diamond-like Carbon Films", Mater. Sci. Forum 52 & 53 (1989), pp. 1-40, have proposed categorization of a-C and a-C:H materials into those without hydrogen (a-C) and those with hydrogen (a-C:H) incorporated into the film. Because the range of properties of a-C and a-C:H materials overlap, categorization by chemical bonding has been suggested. Carbon atoms can individually assume either a three-fold, planar bonding pattern, (sp.sup.2 hybridization) or a four-fold, tetrahedral bonding pattern (sp.sup.3 hybridization). Graphite has three-fold, sp.sup.2 bonding whereas diamond has four-fold, sp.sup.3 bonding. In a-C and a-C:H materials, these two types of bonding are combined in an intimately mixed amorphous structure. In carbon polymers, hydrogen allows termination of carbon one-dimensional chains, two-dimensional sheets and three-dimensional blocks by simple single bonds to hydrogen. This introduces new ranges of elastic and plastic mechanical properties and is available in hydrogenated a-C:H. Therefore, by controlled combination of diamond, graphite and polymer types of bonding, a wide range of material properties can be obtained.
To obtain a-C:H films Mirtich et al (U.S. Pat. No. 4,490,229) uses two dual gridded Kaufman-type ion sources in which one ion source emits a combined argon/hydrocarbon ion beam at .apprxeq.100 eV and large current density while a required second ion source emits an argon ion beam at 200 to 500 eV and small current densities. Mirtich et al describes an initial phase of growth of a-C:H films. Thereafter, the ion current density is increased after the initial phase of growth is complete.
Commonwealth Scientific Corp. (CSC) of Alexandria, Va., Assignee of the present application, manufactures and sells gridless end-Hall ion sources, e.g., Mark I, Mark II, Mark III, for depositing oxide and nitride films into various substrates.
CSC as well as other companies also sell dual gridtied Kaufman-type ion sources, the source described by Mirtich et al. The differences between the principles of operation of the gridless end-Hall and the dual gridtied Kaufman-type ion sources are summarized by H. R. Kaufman et al, in Operation of Broad-Beam Ion Sources, Commonwealth Scientific Corp., Alexandria, Va., (1984), pp. 5-76. It is believed that these differences produce fundamentally different mechanisms of electron transport and ion acceleration causing different deposition characteristics.
The deposition of a-C:H using a Hall effect ion source is described by M. Okada et al, in "Application of a Hall Accelerator to Diamondlike Carbon Film Coatings", Japan J. Appl. Phys. 31 (1992) pp. 1845-1854. However, the type of Hall effect ion source used by Okada et al is as a "closed-drift" source, as distinguished from the end-Hall source according to the present invention. The operational differences between the two mentioned sources are summarized by R. Robinson et al in "Hall Effect Ion Sources" in Handbook of Ion Beam Processing Technology, J. J. Cuomo et al (eds.), Noyes Publications, Park Ridge, N.J. (1989) pp. 39-57. Simply stated, the Okada et al closed-drift source is considerably different than the end-Hall source of the present invention, as will be discussed more fully hereinafter.
From the aforementioned, it will be seen that up until the present invention persons skilled in the art failed to recognize the advantages of utilizing the gridless, end-Hall source for depositing carbon based films on substrates. Nor did the prior art recognize that such utilization of an end-Hall source would produce carbon based coated substrates with good electrically conductive and electron-emissive properties.